1. Technical Field
The present invention embodiments pertain to reduction of extraneous signals within electrical elements. In particular, the present invention embodiments pertain to electrical filters with one or more inductive units each including at least two inductive elements connected in series and arranged to cancel current within the inductive elements induced by vibrations and/or sound pressure (or acoustic) waves, thereby reducing extraneous signals (e.g., signals produced from microphonic behavior, noise, etc.).
2. Discussion of Related Art
In secured facilities, modern networked digital telephone systems (e.g., Voice over Internet Protocol (VoIP) systems) eliminate many of the security concerns with older telephony systems. This is primarily due to the potential to encrypt the voice transmission “end-to-end” beginning at the handset of the telephone device. Although the intentional, packetized voice signal is protected by encryption, limitations of the VoIP equipment design may result in measurable unintended emissions of the original unencrypted (or plaintext) analog voice signal onto a computer network cable. Since almost any inductive component in a telecommunications or computer networking device may exhibit microphonic behavior (e.g., emissions of the conveyed signal) in response to acoustic vibrations, unintended voiceband emissions of plaintext may be present on a network interface when a call is not in progress, or even during the interval equipment is powered down.
Accordingly, telephony equipment intended for installation in secured facilities is tested for immunity to this potential security breach by exposing the equipment to test tones (at voice frequencies from a loudspeaker and at calibrated sound pressure levels), and measuring the electrical signal level present at the corresponding frequency on a network interface cable. In the event of a testing failure, the equipment may be redesigned to pass the test, thereby significantly increasing the costs and complexity of the equipment.
In the event electronic circuitry is utilized to correct the deficiencies, certain circuit elements may be susceptible to microphonic behavior (e.g., emissions of the plaintext or conveyed signal that may be in the form of perceivable voice or audio), thereby producing extraneous signals (e.g., signals produced from microphonic behavior, noise, etc.). A source of these extraneous signals (e.g., voice or audio signals produced from microphonic behavior, noise, etc.) in electronic circuits includes vibration present in the operating environment. For example, mechanical vibration and sound pressure (or acoustic) waves may initiate microphonic behavior and produce the extraneous signals in a circuit exposed to those items. Ferrous core inductors or coils are among circuit elements that are most sensitive to vibration and sound pressure waves. These stimuli may excite vibration of inductor windings with respect to the fixed core, and induce an unwanted alternating current (representing the extraneous signals (e.g., audio or electrical noise)) in the coil with the same frequency as the inducing vibration force. Although the inductor windings may be encapsulated in a rigid compound (e.g., epoxy resin) to prevent movement of the windings relative to the core, this construction is often not available “off the shelf”, or is more costly to produce.